Various proposals have been made for new iron-making processes that substitute existing blast furnace and smelting reduction processes. These proposals relate to the molten metal manufacturing processes for obtaining molten metal, involving pre-reducing metal oxide agglomerates with carbonaceous material in a rotary hearth furnace to form reduced agglomerates and melting the reduced agglomerates in an arc furnace or a submerged arc furnace (e.g., refer to Patent Documents 1 to 4).
However, in the processes that use an electric arc as the melting furnace, the metallization of the reduced agglomerates must be maintained at a high level and the fines ratio must be retained at a low level to ensure: high melting efficiency, refractory protection, suppression of excessive foamy slag formation, and the like. Thus, according to these processes it has been difficult to increase the productivity of rotary hearth furnaces while maintaining higher metallization and retaining lower fines generation. The problem that the facility tends to be large in size is unresolved.
In contrast, according to the processes that use a submerged arc furnace as the melting furnace, the reduced agglomerates form layers in the submerged arc furnace, and damage on refractories and excessive foamy slag formation are less problematic; also limitations on the metallization and fines ratio of the reduced agglomerates are few, and the size of the rotary hearth furnaces can be made relatively smaller compared to the processes that use arc furnaces. However, according to these processes, it is difficult to effectively use the chemical energy of CO gas generated by the reduction of metal oxides remaining in the reduced agglomerates; hence, there remains a problem that the productivity cannot be sufficiently increased and the operation cost cannot be sufficiently reduced.
In methods for manufacturing molten metal using a submerged arc furnace it is possible to omit pre-reduction in a rotary hearth furnace and directly charge unreduced metal oxide agglomerates with carbonaceous material into the submerged arc furnace so that the pre-reduction step and the melting step are performed in the same furnace. However, when the metal oxide agglomerates with carbonaceous material contain volatile metal elements in addition to nonvolatile metal elements that form molten metal (in other words, when iron mill dust is used as the raw material metal oxide), the volatile metal elements that had been evaporated and removed from the reduced agglomerates in the lower region of the furnace re-condense in a low-temperature zone in the upper region of the furnace and circulate in the furnace by adhering to the reduced agglomerates or forming accretions on the furnace walls. Thus, it is possible that the volatile metal elements cannot be efficiently recovered from exhaust gas but also operational problems such as that the reduced agglomerates do not descend property may occur.
Accordingly, in the existing processes, two steps (a pre-reduction step using a rotary hearth furnace and a melting step using a melting furnace) must be provided irrespective of whether an arc furnace or a submerged arc furnace is employed as the melting furnace. These processes require equipment or facilities for transferring the reduced agglomerates from the rotary hearth furnace to the melting furnace as well as two exhaust gas processing lines, i.e., one for the rotary hearth furnace and one for the melting furnace. Thus, the facility cost increases, the thermal loss increases, and the energy consumption cannot be sufficiently decreased as total system or process.